1. Field
The disclosed method and apparatus relates to wireless communications, and more specifically to wireless systems that employ signal acquisition assistance window data to assist a receiver in acquiring selected signals.
2. Related Art
The communications field requires accurate position information in many instances for mobile stations (MSs) such as cellular telephones, personal communication system (PCS) devices, and other user equipment (UE). Global Positioning Systems (GPS) offer an approach to providing wireless MS position determination. These systems employ satellite vehicles (SVs) in orbit around the earth. A GPS user can derive precise navigation information including three-dimensional position, velocity and time of day through information gained from the SVs.
Position measurements using GPS are based on measurements of propagation delay times of GPS signals broadcast from the orbiting SVs to a GPS receiver. The precise capabilities of the GPS system are maintained using on-board atomic clocks for each satellite, in conjunction with tracking stations that continuously monitor and correct satellite clock and orbit parameters.
Each GPS SV transmits two direct-sequence-coded spread spectrum signals in the L-band. Unique pseudo-noise (PN) codes of 1023 bits, or “chips”, per code period for each SV transmitted every 1 millisecond allow the GPS receiver to distinguish which satellite transmits a given code. A 50-bit/sec data stream containing system status information and satellite orbit parameters, useful for the navigation calculations, are also modulated onto each carrier.
The GPS receiver removes the spreading effect of the PN code modulation from each signal by multiplying it by a time-aligned, locally generated copy of the code. This is referred to as despreading. Because the appropriate time alignment, or “code phase,” (effectively, the SV signal time of arrival) is unlikely to be known at receiver start-up, it must be determined by searching during the initial “acquisition” phase of GPS receiver operation.
After despreading is performed, each signal consists of a 50-bit/sec phase shift keyed (PSK) signal at an intermediate carrier frequency. The exact frequency of this PSK signal is uncertain due to the Doppler effect caused by relative movement between the satellite and the MS, and due to local receiver GPS clock reference errors. A search for the Doppler frequency must be performed during initial signal acquisition, because it is usually unknown prior to signal acquisition. A search for code phase offset also occurs within the 1023 chip window, similar to the search for Doppler frequency.
A bit synchronization loop derives data bit timing, and the data stream is finally detected. A position calculation may be undertaken once the signals from at least four satellites have been acquired and locked onto, the code phase measurements have been made, and a sufficient number of data bits (enough to determine the GPS timing reference and orbit parameters) are received. A velocity calculation may be undertaken if a sufficient number of Doppler measurements are also available.
Signal acquisition suffers the disadvantage of requiring a great deal of time and/or hardware resources. The GPS receiver must search across all satellite PN sequences, all code phase hypotheses, and all Doppler frequency offsets in order to locate SV signals. This means searching over up to 32 SVs, 1023 code hypotheses, and 10 kHz of frequency offset. Examined sequentially, signal acquisition can take several minutes. One method of reducing the signal acquisition time is to use parallel signal acquisition hardware, at higher costs, size and power consumption.
In order to reduce signal acquisition delay, information may be provided to aid a receiver in acquiring an SV signal. Such acquisition assistance (AA) information permits a receiver to narrow the space that must be searched in order to locate a signal. This AA data generally consists of expected Doppler information and expected code phase information. Doppler values change with potential MS motion, so while expected Doppler values for each SV can be determined quite precisely for a given location, Doppler uncertainty windows are generally set by assumptions about the range of potential MS motion and rely relatively little upon knowledge of MS position. Expected code phase information generally consists of an expected code phase for each SV and a code phase window of fixed size, where the MS is expected to search.
Systems in which receivers locate ranging signals for position location upon demand, such as SV GPS signals, with the assistance of information provided from another source within the system, are generally referred to as “wireless assisted position location” systems. One example of a wireless assisted position location system is an MS with a GPS receiver, communicating with one or more base stations (BSs) in communication with a core communication network. The MS also communicates with a position determination module (PDM) that provides signal AA data to the MS.
Even with AA data code phase and Doppler windows, the MS may not always acquire the SV signal. This may occur because the SV signal was too weak or corrupted due to noise. It is also possible that an AA data window provided by a PDM was incorrectly located in time (code phase) and/or frequency. For example, the code phase window provided by a PDM may have instructed the MS to search for the SV signal in an incorrect portion of the entire 1023 chip code. The AA data window may have been too small thereby preventing the MS from searching at the correct time and/or frequency. Alternatively, the AA data window may have been too large thereby requiring a large amount of search time. In that instance the allotted time allowed to the MS for searching could expire before acquiring the SV signal.
Typical wireless assisted position location systems do not monitor the quality of AA data at the measurement level, but instead look at the overall wireless system performance in solution space. A need exists for a system and method that monitors AA data quality, in particular the fit of acquisition assistance windows provided by a PDM to an MS.